A dock leveler (dockboard) is a device designed to bridge the gap between a loading dock and the top surface of a truck or trailer bed. Because the truck bed and the top of the loading dock are often not at the same height, most dock levelers can accommodate a range of vertical dock/truck bed mismatches.
Pit dock levelers reside in a pit formed in the dock and usually can accommodate a comparatively large dock/truck bed height mismatch, ±12 inches in most models. Pit dock levelers provide unobstructed access to the dock, but tend to cost more than other dock leveler types, both in initial cost and in the cost of concrete work for their installation within the dock.
Pit dock levelers come in many variations, but most existing pit levelers have a base frame that is secured to the dock pit and upon which other components of the leveler are attached, either directly or indirectly. Pivotally connected to the base frame is a dock platform capable of supporting a substantial load (i.e., fork truck traffic). Pivotally connected to the other end of the platform is a dock platform lip that hangs substantially downwards when not in use and is extended when in use to rest on a truck bed, creating a bridge between the dock and the truck.
The weight of the dock platform/lip combination is not trivial, and if the movements of the platform and lip are not motorized by either hydraulics, screw drives, pneumatics, or the like, then a counterbalancing method using springs is used. In most cases, the counterbalancing mechanism is upwardly biased, and the counterbalancing moment is slightly more than the moment required to rotate the platform upwards, thus  rotating the platform upwards if left unrestrained. Preventing the platform from rotating upwardly is a hold down mechanism.
Typical operation of mechanical upwardly-biased levelers involves pulling a cable or chain to release the hold down mechanism thereby allowing the counterbalance mechanism to rotate the platform upwardly. Towards the top of the platform's upward travel, a chain or cable connected to a mechanism extends the dock platform lip (rotates it away from the platform) in some levelers. In other levelers, the dock platform lip extends when an operator walks down the dock platform when it has reached the top of its upward travel. In either case, an operator releases the cable or chain, re-engaging the hold down. The operator then walks down the platform causing the extended dock platform lip to rest upon the truck bed. The hold down works to allow easy downwards movement of the platform while restricting upward movement of the platform until the operator pulls the releasing cable or chain.